AC and DC gas plasma panel displays including a pair of dielectric plates, each having a pattern of parallel electrodes, are well known in the technical art. Electroluminescent displays similarly constructed but with an electroluminescent material in place of the gas plasma, are also well known.
In the conventional AC driven case, a dielectric layer is deposited over the electrodes to store charge and promote the effective operation of the display. The dielectric plates are parallel to one another, and the electrode patterns are orthogonal with respect to one another in the conventional case.
Furthermore, the front dielectric plate is transparent, permitting light to pass to the forward viewer. However, rearwardly directed light from the luminous discharge is largely lost, since it departs through the rear of the display. Additionally, a small portion of the light is reflected from the distant surface of the rear plate, which undesirably offers a secondary image to the viewer, and in effect tends to confuse him with regard to the image he actually desires to view.
Furthermore, the electrodes are typically metallic and opaque in order to provide high conductivity, and are typically highly reflective. The high reflectivity is a fundamental aid to display brightness, and therefore to legibility. In fact, the brightest portion of each perceived "pixel" or picture element (located at the projected intersection of a pair of orthogonal activated electrodes) is the bright reflection of the luminous discharge from the rear electrode. Nevertheless, the width of the electrodes, and therefore of this bright region, is typically smaller than the eye can resolve at normal viewing distances. As a result, the perceived brightness of the pixel is the average of this bright region and the other dimmer region within the resolution dimension of the eye. This is undesirable; the bright region should preferably be even larger than this resolution dimension, for good contrast in high ambient illumination.
The width of the electrodes is not made larger in typical flat panels for several reasons: (1) ordinarily, high resolution is required for these displays; thus electrode spacing needs to be as close as feasible; (2) electrical crosstalk between adjacent electrodes is desirably eliminated by making the nonconductive space between them sufficiently large; (3) some flat panel applications require sufficient transparency to view objects (such as a map) behind the display, and therefore a large transparent region between electrodes is desirable.
One alternative approach is to make the electrodes transparent, rather than opaque. However, this is not compatible with the high conductivity required for such electrodes, and would limit the resulting displays to very small sizes.
Accordingly, it is an object of the invention to improve the brightness and contrast of flat panel displays for use in environments in which the ambient illumination level is relatively high by preventing wasteful disposition of a portion of the light.